The invention relates to a method of preparing an aluminium-containing iron oxide nucleus with an xcex1-FeOOH crystal structure from FeCl2. This nucleus is a suitable starting material for the preparation of iron oxide yellow and for use as a yellow coloring pigment.
Synthetic iron oxides are usually prepared by the Laux method, the Penniman method, the precipitation method, the neutralisation method or the roasting method (Ullmann""s Encyclopedia of Industrial Chemistry, 5th edition, 1992, Vol. A20, pages 297 et seq.). The iron oxides thus obtained are generally employed as pigments.
Basically two processes are known for preparing fine-particle xcex1-FeOOH (needle width between 5 and 30 nm):
the acid process and
the alkaline process.
In the acid process an iron (II) component, an iron salt dissolved in water, is introduced initially and an alkaline component is added to it in metered quantities and with intensive mixing; the alkaline component is generally an alkali metal or alkaline earth metal compound dissolved or suspended in water or an ammonia solution. The quantity of alkaline component added is generally between 15% and 70% of the stoichiometrically required quantity. The pH after its addition is within the slightly acid range.
When the addition of the alkaline component is over the material is oxidized with an oxidant, usually atmospheric oxygen. The reaction is carried out at temperatures between 20 and 50xc2x0 C. At considerably higher temperatures there is a danger of undesirable magnetite forming. The end of the reaction can be recognized by a sharp drop in pH and redox potential. When the reaction is over the properties of the product obtained (generally described as the nucleus) are determined and the productxe2x80x94if suitablexe2x80x94is processed immediately to form an xcex1-FeOOH pigment.
The alkaline process differs from the acid process in the quantity of alkaline component. In the alkaline process it is at least about 120% of the stoichiometrically required quantity and generally considerably more. The temperatures at which the reaction is carried out may be slightly above those used in the acid process as there is less danger here of magnetite forming.
The alkaline process basically produces relatively long-needled xcex1-FeOOH crystallites with a length to width ratio of 10:1 to 30:1. As these crystallites also contain very few dendrites the process is particularly suitable for preparing xcex1-FeOOH as a starting material for magnetic tapes.
Nuclei prepared by the alkaline process cannot be employed directlyxe2x80x94or only to a limited extentxe2x80x94for making (xcex1-FeOOH pigments for use in paints and lacquers, as all the other coloring metals present in the industrially used Fe component become incorporated during the process. These metals (particularly Mn, Cr, Cu, Ni) have a considerably adverse effect on color properties, thereby restricting the use of nuclei thus prepared for the production of coloring pigments.
In the preparation of iron oxide pigments it is preferable to start with an xcex1-FeOOH nucleus and then to coarsen it (build it up) in the acid substance, thereby diminishing the introduction of coloring metals. Furthermore the build-up must only take place at pH levels below about 4, as coloring metals are introduced to an increasing extent at higher pH levels. Moreover, the particle shape of the (xcex1-FeOOH considerably affects the color properties, the viscosity of the lacquer and the requirements for the binder.
Short-needle xcex1-FeOOH particles are necessary in order to obtain the desired low viscosity in the lacquer and little binder requirements. These can be made by intensively grinding long-needle xcex1-FeOOH particles. A more favourably priced alternative is direct preparation of short-needle xcex1-FeOOH particles.
Modifying additives are required in order to guide the particle shape of the xcex1-FeOOH nucleus and thus that of the pigment built up therefrom towards a low length to width ratio. The use of B, Al, Ga, Si, Ge, Sn or Pb as nucleus modifiers is known from U.S. Pat. No. 4,620,879. This patent specification describes an iron oxide yellow with a particularly low Silking index, achieved through an appropriate pigment build-up procedure and addition of the modifiers listed above. Although the specification describes the use of FeCl2 it does not mention the exact conditions for preparing an (xcex1-FeOOH nucleus from FeCl2. However since FeCl2 clearly differs from FeSO4, especially in the nucleus-forming phase, the conditions under which a good pigment is obtained with FeSO4 cannot be transferred to FeCl2.
An object of the invention was to provide a method of preparing a short-needle xcex1-FeOOH nucleus by the precipitation process in a simple and cost-effective way. In a further step the xcex1-FeOOH nucleus is built up to form an xcex1-FeOOH pigment.
The object is solved by the method of the invention.
The invention relates to a method of preparing aluminium-containing iron oxide nuclei with an xcex1-FeOOH crystal structure with an aspect ratio (AR) of 2100 to 3600 and a BET surface area of 50 to 150 m2/g using FeCl2, comprising the steps of
a) initially adding 4-13 mol %, based on the total iron, of an Al component to an iron (II) chloride solution with a total Fe content of 20-100 g/l, and an Fe (III) content of 0.1 to 10 mol % Fe (III) (based on the total Fe)
b) heating the mixture to a precipitation temperature between 30 and 60xc2x0 C.,
c) adding a precipitating agent with an active ingredient content of 2-10 equivalents per liter, to the mixture and the molar ratio of Fe+Al to precipitating agent is 20-80% of the stoichiometric quantity,
d) oxidizing the precipitated suspension by an oxidant at a speed such that the oxidation rate is 2-50 mol %/h of the iron to be oxidized.
In the context of the present invention the aspect ratio is the product of the BET surface area and the mean crystallite size, determined from the 110 reflex of the xcex1-FeOOH by X-ray crystallography.
The Al-containing xcex1-FeOOH nucleus obtained after oxidation may optionally by used to prepare iron oxide yellow pigments without further separation, when its properties have been tested.
The process preferably uses
a) an iron (II) chloride solution with a total Fe content of preferably 40-65 g/l,
b) a precipitation temperature between 35 and 50xc2x0 C.,
c) a precipitating agent with an active ingredient content of 4-8 equivalents per liter,
c) a molar ratio of Fe+Al to precipitating agent of 30-60% of the stoichiometric quantity, and
The Al-containing xcex1-FeOOH nucleus obtained after oxidation is separated if appropriate.
It is more preferred to proceed as follows:
Initial chemicals:
FeCl2 solution with an Fe content of 55 g/l Fe, including 1.5 mol % Fe (III)
AlCl3 solution
NaOH solution with an NaOH content of 300 g/l=7.5 equivalents NaOH/l
Al/Fe ratio: 12-13
Proportion of Fe+Al/precipitating agent: 35-40%
Reaction conditions:
Temperature: 44xc2x0 C.
Oxidation speed: 30-35 mol % Fe (II)/h
AlCl3 (as an aqueous solution) is preferably employed as the Al component. The use of Si or Ti as nucleus modifiers, in their chloride form, is also possible but entails a greater industrial outlay on production.
NaOH, KOH, Na2CO3, K2CO3, Mg(OH)2, MgO, MgCO3, Ca(OH)2, CaO, CaCO3, NH3 or secondary or tertiary aliphatic amines may be used as precipitating agents in an aqueous solution or an aqueous slurry.
The oxidant used may be atmospheric oxygen, oxygen, ozone, H2O2, chlorine, nitrates of alkali metals or alkaline earth metals or NH4NO3.
If the iron (II) chloride solution employed contains quite large quantities of precipitable coloring metals at pH levels below 4, these may be precipitated by adding an alkaline component to the iron (II) chloride solution up to pH 4. The solid formed may be separated from the remaining clear, purified solution by sedimentation, filtering or centrifuging. Not only are the undesirable coloring metals removed but also Fe (III), which has a seriously undesirable effect on the reaction to form the xcex1-FeOOH nucleus (it forms black magnetite).
The reaction takes place in discontinuous or continuous agitating boilers, in cascades of agitating boilers, loop-type reactors or reactors without agitators and with dual-material nozzles as mixing members.
When the xcex1-FeOOH nuclei according to the invention have been prepared they are converted to a pigment by the known process of coarsening the nucleus particles (building up the pigment). But as the xcex1-FeOOH nuclei according to the invention are not used as such it is necessary to describe how the pigment is built up to form an iron oxide yellow pigment.
The Al-containing nucleus prepared by the method of the invention is pumped to a solution of FeCl2 or FeSO4 or a different Fe (II) salt. 7-15 mol Fe (II) salt, as a solution with an Fe content of 30-100 g/l Fe, is used per mol of FeOOH in the nucleus. Addition of larger quantities of Fe (II) salt does also produce xcex1-FeOOH yellow pigments but the brightness diminishes as the quantity of added Fe (II) salts is increased, which is generally undesirable. The suspension is heated to the reaction temperature, which is between 50 and 90xc2x0 C. When the precipitation temperature has been reached oxidation and precipitation are started simultaneously. Atmospheric oxygen is generally added through a suitable gas supplying device and the pH of the suspension is adjusted with an alkaline precipitating agent. The pH is adjusted within the 2.4 to 4.8 range. The oxidation speed should be between 0.5 and 8 mol % Fe (III)/h.
When the reaction is over (i.e. when all the Fe (II) has been oxidized) the solid formed is filtered off. It is washed to remove the salt and can then be dried.
The yellow pigment made by this method is distinctive in its very pure color, almost isometric particle shape, low oil absorption number and high chemical purity. The sum of its properties makes it particularly suitable for:
application in the field of paints and lacquers
applications as a raw material for catalysts
applications in the food coloring field
applications in the paper coloring field
applications in the polymer coloring field
applications as a UV stabiliser
applications in the field of high-grade building materials (plasters etc)
applications in the field of dispersion colors
Due to the cost-effective raw materials and the high production rate in preparing the yellow pigment the process is particularly economical. Due to the special reaction process and use of a precisely specified nucleus one can prepare especially high-grade yellow pigments which are more advantageous to use than pigments made by other methods.
Environmentally relevant chemicals are not employed in preparing the yellow pigments according to the invention.
In the preferred embodiment (with FeCl2, AlCl3, NaOH and air used as the ingredients) an almost closed circuit is possible through electrolysis of the NaCl obtained as a by-product. The resultant sodium hydroxide solution may be re-used directly in the process. The H2 and Cl2 produced by the chlor-alkali electrolysis can be converted to HCl, which can in turn be used for etching the steel plates. This particularly environmentally friendly technology is not at present possible with FeSO4 as electrolysis of FeSO4 does not work satisfactorily.
Description of the Measuring Methods Used
1. Measuring the BET Surface Area
The BET surface area is determined by the so-called one point method to DIN 66131. 90% He and 10% N2 are used as the gas mixture and the measurement is taken at 77.4 K. The sample is heated for 60 minutes at 140xc2x0 C. before measuring.
2. Radiographic Measurement of the Crystallite Size
The crystallite size is determined on the Phillips powder diffractometer. The 110 reflex is used for this purpose.
xcex1-iron oxide hydroxide (M(FeOOH)=88.9 g/mol
2.1 Field of Application
Determining crystallite size in goethite within the 5 to 100 nm range.
2.2 Basic Method
The size in goethite is determined after X-ray diffractometric radiation through reflection detection. The evaluation is made using silicon as an external standard.
2.3 The Reagent
Silicon standard for angular calibration (ICDD no. 27-1402), Philips PW 1062/20